This application is a continuation of co-pending U.S. patent application Ser. No. 13/327,744, filed on Dec. 15, 2011, which is a divisional of U.S. patent application Ser. No. 11/173,620, filed on Jul. 1, 2005, which claims priority to U.S. Provisional Patent Application No. 60/585,365, filed on Jul. 2, 2004.
At least certain embodiments of the present inventions relate to pigments, inks, and other materials which may be applied onto media, such as paper, or other materials. These systems and methods may be used as security features or identification features to allow an object to be authenticated.
Existing methods that use UV (ultraviolet) light to activate a visible effect produce only a static emission spectrum. Such features rely on fluorescence in which UV light is absorbed by a material and then re-radiated as a longer wavelength, typically in the visible spectrum. The emitted light is determined by the chemical and electronic structure of the compound and is a material property thereof. The spectrum of the light thus induced does not change under steady-state illumination or under intensity-varying conditions.
Existing methods also include the use of a photochromic material in authentication systems, such as security printing inks. See, for example, Organic Photochromic and Thermochromic Compounds, Volume 1, edited by John C. Crano and Robert J. Gugoielmetti, 1999 (ISBN 0-306-45882-9).